Potassium channels belong to a ubiquitous and heterogeneous protein family, selectively permitting K+ ions to move across the cell membrane. These channels play an important role in adjusting cellular excitability through maintenance of the optimum set of conditions for K+ ion and its various effects on membrane potential and membrane resistance. There are different kinds of K+ channel in human body, which are activated by different mechanism. By considering the different factors influencing on the activity of different classes of K+ channels, calcium-activated potassium channels are the one, of which opening was determined by the raise of the intracellular calcium concentrations, and regulated by transmenbrane voltage and phosphorylation states.
Calcium-activated potassium channels are further divided into three major types, which can be distinguished electropysiologically by their different single-channel conductance.
The BKCa (or Maxi-K) channel has the particular function of large single-channel conductance (100˜250 pS), whereas other two major types of calcium dependent potassium channels are small conductance 92˜25 pS, SKCa) and intermediate conductance group (25˜100 pS, IKCa). Among these three kinds of channels, BKCa channel is particularly regarded because of the extensive K+ efflux and membrane hyperpolarization due to their large single-channel conductance, and their expression in a range of non-excitable and excitable cell types including neurons and muscles. Especially, BKCa channels play roles in shaping action potentials and regulating neuronal excitability and neuro-transmitter release in nervous system.
The therapeutic potentials of BKCa channel are more evident in pathological conditions such as potential neurotoxic cascade introduced by excess Ca2+ entry, which could be limited or interrupted by BKCa channel activators or openers. Thus, designing chemical openers of BKCa channel would be a strategy for the development of drugs to treat neuronal damages resulted from traumatic and ischemic events or neuro-degenerative processes.
Also, the vaso-relaxation effects of BKCa channel openers could be utilized to develop drugs to treat cardiovascular diseases or hypertension, airway smooth muscle related disease such as asthma and erectile dysfunctions.
BKCa channels are composed of two different subunits: the pore forming α subunit and the auxiliary β subunits. Although channels formed only by four α subunits can be functional, β subunits alter the biophysical and pharmacological properties of homomeric channels, including Ca2+ and voltage sensitivity, and gating kinetics.
Several compounds have been developed and reported to be BKCa channel openers (e.g., dehydrosoyasaponin-I, maxikdiol, NS-1619, BMS-204352, 17-β-estradiol, ethylbromide tamoxifen, pimaric acid and epoxyeicosatrienoic acid). Although some synthetic activators, such as, NS-1619 and BMS-204352, act on the α subunit, other openers of BKCa channels, including dehydrosoyasaponin-I and 17-β-estradiol, require β subunit for their action. Several activators derived from natural products such as dehydrosoyasaponin-I are impermeable to the cell membrane and act only on intracellular side of BKCa channels.
In U.S. Pat. No. 6,288,099, the skeleton of benzofuroindole and its derivatives have been disclosed as calcium channel opener. However, in this disclosure, only whole skeleton of benzofuroindole and its broad derivatives have been suggested without specifying the selective substituent.
Among the prototypical BKCa channel openers 1 (BMS-204352), quinolinone analog 2 and substituted benzofuroindole analog 3, compound 1 was reported to selectively shift the BKCa channel activation curve toward less positive membrane potentials in a dose dependent manner and currently in clinical trials targeting acute ischemic stroke.
Benzofuroindole analog 3 was studied as a BKCa channel opener to show bladder selective smooth muscle relaxation effect and increasing outward current in voltage clamp experiment on isolated rat bladder smooth muscle cells. However, no further results have been reported regarding the structural activity or biological property in the duplicated BKCa channel.